Synchronized binaural hearing system

ABSTRACT

A wireless binaural hearing aid system that utilises direct sequence spread spectrum technology to synchronize operation between individual hearing prostheses is provided.

RELATED APPLICATION DATA

This application is a re-issue application of U.S. patent applicationSer. No. 10/342,625, issued as U.S. Pat. No. 6,839,447, which is acontinuation of International Application No. PCT/DK01/00493 filed onJul. 13, 2001, which claims priority to and the benefit of Danish PatentApplication No. PA 2000 01094 filed on Jul. 14, 2000.

FIELD OF THE INVENTION

The present invention relates to a binaural hearing system thatcomprises two fully or partly synchronously operating hearing prosthesescapable of performing bi-directional data communication over a wirelesscommunication channel. Fully synchronous operation between the hearingprostheses is preferably maintained by utilising direct sequence spreadspectrum technology to lock all clock signals of a slave hearingprosthesis to a coding clock signal provided by a clock oscillator inthe master hearing prosthesis during the bi-directional datacommunication. Thus, simultaneous sampling of respective microphonesignals of the hearing prostheses is obtained so as to provide awireless binaural hearing system that supports binaural signalprocessing techniques and algorithms.

BACKGROUND OF THE INVENTION

Hearing aid systems with bi-directional communication capability arewell known in the art. U.S. Pat. No. 5,991,419 discloses a so-calledbilateral hearing instrument that comprises two units for placement in ahearing aid user's left and right ears, respectively. Each instrumentcomprises an associated transceiver circuit so as to providebi-directional wireless communication between the instruments. WO99/43185 discloses a resembling binaural digital hearing aid systemadapted to exchange raw or processed digital signals between two hearingaids to allow each aid to perform a processing of its own input signalas well as a simulated processing of the processing performed in theother aid, i.e. the hearing aid that is arranged on the user's reverseside. The simulated processing of reverse side signals is performed toprovide a binaural signal processing technique that can restore binauralsound perception by taking into account differences in hearing loss andcompensation between the user's two ears. U.S. Pat. No. 5,751,820discloses an integrated circuit design for bi-directional communicationutilising reflective communication technology to obtain low powerconsumption, thereby making the design suitable for battery operatedpersonal communication systems, such as binaural digital hearing aidsystems.

However, while it has been noted in the above-mentioned prior art that apractical binaural hearing aid system must have control of thesynchronisation between the ear units, and that U.S. Pat. No. 5,991,419states that the phase error between the units should correspond to timeerrors less than 10 μS, there has not been disclosed an adequatewireless synchronisation technology that would actually be capable ofproviding the required synchronisation between the units or aids.

To perform correct binaural processing of the respective signals of suchbinaural hearing aid systems it is mandatory to assure that theindividual hearing aids or instruments are operating synchronously withrespect to each other. In particular, the respective microphone signalsmust be sampled substantially synchronously to enable e.g. binauralbeamforming and off-axis noise cancellation. Time shifts as small as20-30 μS between sampling instants of the respective microphone signalsin the two hearing aids may lead to a perceivable shift in the beamdirection. Furthermore, a slowly time varying time shift between thesampling instants of the respective microphone signals, which inevitablywill occur if the hearing aids are operated asynchronously, will resultin an acoustic beam that appears to drift and focus in alternatingdirections. An undesirable effect, which certainly will be very annoyingfor the hearing aid user.

Consequently, in order to provide a practical binaural hearing system itis highly desirable to provide a wireless communication technique thatassures synchronised operation between the individual hearing prosthesesand which, at the same time, is practical for miniature and low-powerbattery operated equipment such as hearing prostheses.

DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a binaural hearing systemcomprising a first and a second hearing prosthesis adapted for wirelessbi-directional communication of digital data signals; the first hearingprosthesis comprises a first microphone adapted to generate a firstinput signal in response to receiving acoustic signals,

-   -   a first analogue-to-digital converter adapted to sample the        first input signal by a first sampling clock signal to generate        a first digital input signal,    -   a first clock generator adapted to generate a coding clock        signal, a data rate clock signal and the first sampling clock        signal synchronously with respect to each other,    -   a first sequence generator adapted to generate a repetitive        coding sequence synchronously to the coding clock signal,    -   first data generating means adapted to provide a first data        signal synchronously to the data rate clock signal,    -   a first wireless transceiver adapted to receive and modulate the        first data signal with the repetitive coding sequence to        transmit a first modulated data signal to a second wireless        transceiver of the second hearing prosthesis and to retrieve a        second data signal from a second modulated data signal received        from the second wireless transceiver,    -   first output means adapted to convert a first processed data        signal to a first acoustical or electrical output signal.

The second hearing prosthesis comprises a second microphone adapted togenerate a second input signal in response to receiving acousticsignals,

-   -   a second analogue-to-digital converter adapted to sample the        second input signal by a second sampling clock signal to        generate a second digital input signal,    -   a second sequence generator adapted to generate a version of the        repetitive coding sequence of the first sequence generator        synchronously to a second coding clock signal,    -   second data generating means adapted to provide a second data        signal synchronously to a retrieved clock signal,    -   a second wireless transceiver adapted to receive the first        modulated data signal from the first wireless transceiver and to        modulate the second data signal with the version of the        repetitive coding sequence to transmit a second modulated data        signal to the first wireless transceiver,    -   second clock and data retrieval means adapted to lock onto the        first modulated data signal to retrieve the first data signal        and to generate the second sampling clock    -   signal and the retrieved clock signal, synchronously to the        first coding clock signal, by correlating said first modulated        data signal with the version of the repetitive coding sequence,    -   second output means adapted to convert a second processed data        signal to a first acoustical or electrical output signal.        Thereby, the respective sampling clock signals of the hearing        prostheses are synchronised in time so as to provide a hearing        system with synchronous sampling of the respective microphone        input signals.

According to the invention, the first clock generator operates as amaster clock circuit for both hearing prostheses of the binaural hearingsystem during bi-directional communication of the first and seconddigital signals or data signals to ensure synchronous sampling of therespective microphone input signals. By locking the second clock anddata retrieval means onto the received first modulated data signal, itis ensured that the retrieved clock signal and the second sampling clocksignal in the second hearing prosthesis are synchronous to the codingclock signal generated by the first clock generator in the first hearingprosthesis. The microphone signal in the second hearing prosthesis istherefore sampled synchronously to the sampling of the microphone signalin the first hearing prosthesis. Thus, a binaural beam-formingalgorithm, or other types of binaural processing algorithms, executed inthe binaural hearing system are capable of correctly determinedirections to acoustic sources by examining inter-device differencesbetween the digital input signals, such as phase or group delaydifferences.

Frequencies of the synchronous coding and data rate clock signals may beselected to about 9600 kHz and 600 kHz, respectively. The coding clocksignal is used to clock the first sequence generator and the data rateclock signal is preferably used to control a timing of the first datasignal in order to synchronise the repetitive coding sequence to thefirst data signal. The first sampling clock signal is finally alsoderived synchronously to the coding clock signal (and therefore to thedata rate clock signal) to allow the first or master clock generator tocontrol the timing of the sampling of the first input signal. Thesampling clock signal and the data rate clock signal may be derived fromthe coding clock signal by well-known clock division and/ormultiplication methodologies e.g. using D-Flip-Flops, PLLs, etc.

The first and second analogue-to-digital converters are preferably bothof an oversampled sigma-delta type with a sampling frequency of about 1MHz, thus making it possible to avoid analogue lowpass filters tobandwidth limit the first and second input signals provided by therespective microphones before sampling. The first and second digitalinput signals may be represented by respective non-decimated, e.g.single bit format signals, or by corresponding decimated signals havinga sampling rate in or close to the audio-frequeny range, e.g. about 16kHz with a resolution of 1-20 bits such as 16 bits.

The first and second data signals, provided by the respective datagenerating means, may be constituted by the first and second digitalinput signals, respectively, so that substantially unprocessed or “raw”time discrete microphone input signals are transmitted to the otherhearing prosthesis. In this situation, the data rate of each of thefirst and second data signals, during transmission, may be selected toabout 512 Kbit/s. Such a data rate corresponds to representing each ofthe first and second data signals with a sequence of 16 bits samples ata sample rate of 16 kHz during bi-directional communication in atime-multiplexed mode with a transmission duty cycle of 50%.

Alternatively, the first and/or second data signal(s) may bepre-processed digital signals which has or have been derived by theirrespective data generating means that, for the purpose of processing thedata signals, may comprise one or more DSPs. This pre-processing maymodify audio characteristics of the digital input signals such, e.g.filtering and/or compressing one or several frequency bands of therespective data signals.

Preferably, the data generating means are adapted to encode theirrespective data signals prior to transmission in accordance with apredetermined error detection and/or correction scheme. The encodingallows data errors, typically caused by electromagnetic interferencefrom other RF-sources, introduced into the data signals duringtransmission to be detected and/or corrected. The encoding may also beadapted to reduce the data rates of the data signals and/or to remove aDC content of the data signals. A large number of suitable encodingschemes has been disclosed in the relevant literature and will as suchbe well known to the skilled person. Accordingly, this issue will not bediscussed further here. Finally, encoding of the first and/or seconddata signal may implemented by inserting control data in one or both ofthe data signals in order to communicate control data from the first tothe second hearing prosthesis and/or vice versa. The control data may beutilised to support e.g. co-ordination in operation mode between thefirst and second prostheses, e.g. co-ordinate automatic or usercontrolled switching between a number of pre-set listening programsand/or between different audio input sources such microphone input,dual-microphone input, telecoil input, direct audio input etc.

The first and second sequence generators are preferably both adapted togenerate respective versions of an identical pseudorandom noise (PN)sequence. The two PN sequences will be phase-aligned, and synchronous tothe coding clock signal, when the second clock retrieval and generatingmeans have locked onto the first modulated data signal. Sequencegenerators for generating PN sequences are particularly well suited forimplementation in digital circuits where a number of low-power anddie-area efficient implementations are possible. The modulation of thefirst and second data signals with their respective repetitive codingsequences can furthermore be implemented by simple sign encoding ormodulation e.g. by switching the data signals to +1/−1 volt. Signmodulation is particularly convenient to implement in CMOS technologysince CMOS transistors are relatively good switch elements. By applyingthe above-mentioned modulation scheme, the resulting modulation of thedigital signals is commonly referred to as direct sequence spreadspectrum modulation (DS-SS). Alternatively, the first and secondsequence generators may be adapted to control respective frequencysynthesisers controllable to transmit signals on anyone of a pluralityof carrier frequencies. Values of the PN sequence is utilised torandomly select a particular carrier frequency of the plurality ofcarrier frequencies and thus modulate the data signal. Thereby, therepetitive coding sequences will comprise a carrier signal that hopsbetween different carrier frequencies in a pseudorandom manner. Thislatter modulation scheme is commonly referred to as frequency hoppedspread spectrum modulation (FH-SS).

In order to process the first and second input signals with advancedbinaural signal processing algorithms, one of the first or secondhearing prosthesis or both of them may comprise a Digital SignalProcessor. Accordingly, the binaural hearing system may operate ineither a symmetric or in an asymmetric mode. In the asymmetric mode, thedata generating means of the first hearing prosthesis comprise a DigitalSignal Processor(DSP) adapted to process the first digital input signaland the second data signal in accordance with a predetermined signalprocessing algorithm to provide the first processed data signal or viceversa if the DSP is located in the second hearing prosthesis. In thisasymmetric mode, the DSP is preferably adapted to also generate a firstor second data signal that has been binaurally processed and whichtherefore may be passed directly to the output means on the reverse sidehearing prosthesis. Thereby, the asymmetric binaural hearing system mayoperate with a single DSP that processes the digital input signals fromboth hearing prostheses and generate binaurally processed data signalsfor both aids. Naturally, such an asymmetric binaural hearing system maycontain DSPs in both hearing prostheses so that the asymmetric operationis obtained by programming one of the devices as a master device duringthe initial fitting of the binaural hearing system. The master device,in this situation, is programmed to execute the predetermined signalprocessing algorithm to generate and provide respective binaurallyprocessed signals for both hearing prostheses. An advantageous propertyof this latter embodiment of the invention is that the hearingprostheses in a binaural pair can be identical units which may simplifythe distribution and repair handling procedures.

In the symmetric operating mode, the data generating means of the firsthearing prosthesis comprise a first Digital Signal Processor adapted toprocess the first digital input signal and the second data signal inaccordance with a predetermined first signal processing algorithm toprovide the first processed data signal to the first output means. Thedata generating means of the second hearing prosthesis comprise a secondDigital Signal Processor adapted to process the second digital inputsignal and the first data signal in accordance with a predeterminedsecond signal processing algorithm to provide the second processed datasignal to the second output means.

According to a preferred embodiment of the invention, the first DigitalSignal Processor and the first output means operate synchronously to thecoding clock signal, and the second Digital Signal Processor and thesecond output means operate synchronously to the retrieved clock signal.Thereby, the acoustical or electrical output signals of the respectivehearing prostheses are synchronised in time so as to provide a hearingsystem capable of delivering phase aligned acoustic or electrical outputsignals to the user's eardrums. All clock signals within the secondhearing prosthesis are preferably locked to the retrieved clock signal(and thereby to the coding clock signal) while all clock signals withinthe first hearing prosthesis are synchronised to the coding clocksignal. This embodiment of the invention provides a simple and efficientmethod of synchronising all clock signals within the entire binauralhearing system, i.e. also across the wireless communication channel.Such a completely synchronised hearing system supports binauralprocessing algorithms that are capable of retaining naturally occurringbinaural signal cues, such as interaural phase and level differences, inthe acoustic or electrical output signals provided to the user.

For some applications of the present binaural hearing system, it may beadvantageous to make the second hearing prosthesis capable of operatingas a stand-alone device, independently of whether or not the firsthearing prosthesis transmits the first modulated data signal. This hasbeen accomplished by a binaural hearing system wherein the secondhearing prosthesis comprises a second clock oscillator adapted togenerate a second coding clock signal and the second sampling clocksignal. The second hearing prosthesis further comprises clock modeselection means operatively connected to the second clock and dataretrieval means and the second clock oscillator and adapted toselectively use the second clock and data retrieval means or the secondclock oscillator as a source for clock signals in the second hearingprosthesis. Thereby, a mono-aural operation mode is supported by bothhearing prostheses during time periods with interruptions in the firstmodulated data signal.

According to this embodiment of the invention, the second hearingprosthesis is adapted to automatically operate in mono-aural mode if theclock mode selection means detect that the first modulated data signaland/or the first data signal is/are absent or contain(s) too many errorsto be used.

Since it may be impractical to sell and distribute binaural hearingsystems where only one of the pair of hearing prostheses is capable ofoperating as a master device during bi-directional communication, apreferred embodiment of the invention is one wherein the first hearingprosthesis further comprises first clock and data retrieval meansallowing the prosthesis to lock onto the second modulated data signal tosynchronise clock signals of the first prosthesis to the second clockoscillator. In such a binaural hearing system, operation as a masterdevice is supported for both the first and the second hearingprosthesis. In a particularly preferred embodiment of the invention, theselection of which of the hearing prostheses that should operate as themaster(and the other as a slave device) during binaural operation can beselected during the initial fitting session by programming the devicesfrom a fitting system. Each of the hearing prostheses comprises aprogramming interface for exchanging programming data between a hostprogramming system and the hearing prosthesis, and a configurationregister programmable through the programming interface and operativelyconnected to the clock mode selection means to control their operation.

According to yet another embodiment of the invention, the first andsecond modulated data signals are transmitted by their respectivewireless transceivers without having any further RF modulation appliedthan the modulation provided by the repetitive coding sequence. Thisembodiment of the invention has as a particularly attractive featurethat commonly employed RF modulators and demodulators can be dispensedwith to minimise current and area consumption and reduce designcomplexity of the first and second wireless transceivers.

However, for other applications it may be more effective, in particularin terms of minimising power consumption, to include within the firstwireless transceiver a first RF modulator adapted to further modulatethe first modulated data signal to generate and transmit a first RFmodulated data signal to the second hearing prosthesis and a first RFdemodulator adapted to recover the second modulated data signal from asecond RF modulated data signal. The second wireless transceiver furthercomprises a second RF modulator adapted to further modulate the secondmodulated data signal to generate and transmit the second RF modulateddata signal to the first hearing prosthesis and a second RF demodulatoradapted to recover the first modulated data signal from the first RFmodulated data signal from the first wireless transceiver. Thisembodiment may be more power efficient than the direct transmission ofthe first and second modulated data signals since a carrier frequency ofthe RF modulators may be selected so as to provide an optimum match to aparticular type of transmission/reception antennas. Accordingly, in thepresent specification and claims the term “modulated data signal” maydesignate a data or digital signal which solely has been modulated withthe coding sequence prior to transmission. Or the term may designate adata signal that has been modulated with the coding sequence to form acomposite signal and thereafter further modulated or up-converted with aRF carrier signal so as to provide e.g. a FSK modulated RF compositesignal.

The first and second wireless transceivers must comprise some form ofantenna means to transmit/receive the modulated data signals. Forhearing aid applications, it may be difficult to provide sufficienthousing space for an effective RF antenna. This is particularly true ifit is desired to transmit the modulated data signals in the RF rangebelow about 1 GHz due to relatively large wavelengths, in comparison totypical dimensions of hearing aids, of such RF signals.

According to an embodiment of the invention, each of the first andsecond wireless transceivers comprises an inductive coil where theinductive coils are adapted to transmit and receive the modulated datasignals, or the RF modulated data signals, by utilising near-fieldmagnetic coupling between said inductive coils. Each of the inductivecoils may be tuned to a target transmission frequency by arranging asuitable tuning capacitor across the coil so as to provide a Q for eachof the inductive antennas of about 4, preferably between 3 and 10 tooptimise the received/transmitted power at the antennas. Thecommunication frequency is preferably selected to a frequency somewherebetween 50-100 MHz for such a magnetically coupled system.

The above-described binaural hearing system is adapted to communicatebi-directional data signals to support binaural signal processingalgorithms and thereby allow the hearing system to restore or enhancebinaural signal cues in the acoustic input signals.

However, it may also be advantageous to provide a hearing aid systemwhere spread spectrum techniques are employed for the purpose ofsynchronising the signal processing between the hearing aids to securee.g. identical sampling frequencies between the aids. A signal delay orgroup delay through a DSP based hearing prostheses is commonly dominatedby a group delay associated with the digital processing of the inputsignal. This group delay is furthermore substantially proportional tothe inverse of each individual hearing prosthesis' own master clockfrequency. Since a common tolerance on the latter value is about+/−5-10%, the group delay difference between two randomly selectedhearing prostheses may be quite large. Consider a case where aparticular hearing prosthesis has a nominal group delay value of 5 ms.Individual prostheses of the same type may exhibit a group delayanywhere from 4.5 ms to 5.5 ms. The group delay difference between thesevalues is more than the maximum interaural time delay of 600-700 μS thatoccurs in natural, i.e. unaided, human hearing. By providing matching ofthe signal delays through the hearing prostheses, binaural signal cuesin the input acoustic signals can better be preserved.

A second aspect of the invention therefore relates to a wirelesssynchronised hearing aid system comprising a first and a second hearingprosthesis wherein the first hearing prosthesis comprises:

-   -   a first microphone adapted to generate a first input signal in        response to receiving acoustic signals and a first        analogue-to-digital converter adapted to sample the first input        signal by a first sampling clock signal to generate a first        digital input signal,    -   a first clock generator adapted to generate a coding clock        signal and a first sampling clock signal synchronously with        respect to each other,    -   a first sequence generator adapted to generate a repetitive        coding sequence synchronously to the coding clock signal,    -   a first wireless transmitter adapted to transmit a        synchronisation signal based on the repetitive coding sequence        to a second wireless receiver of the second hearing prosthesis,    -   a first Digital Signal Processor and first output means,        operated synchronously to the the coding clock signal, and        adapted to process the second digital input signal in accordance        with a predetermined second signal processing algorithm to        provide a first acoustical output signal; and the second hearing        prosthesis comprises: a second microphone adapted to generate a        second input signal in response to receiving acoustic signals,    -   a second analogue-to-digital converter adapted to sample the        second input signal by a second sampling clock signal to        generate a second digital input signal,    -   a second sequence generator adapted to generate a version of the        repetitive coding sequence of the first sequence generator        synchronously to a retrieved clock signal,    -   the second wireless receiver being adapted to receive the        synchronisation signal and retrieve the repetitive coding        sequence,    -   second clock retrieval means adapted to lock onto the        synchronisation signal to retrieve to generate the retrieved        clock signal and the second sampling clock signal, synchronously        to the first coding clock signal, by correlating said        synchronisation signal with the version of the repetitive coding        sequence,    -   a second Digital Signal Processor and second output means,        operated synchronously to the retrieved clock signal, and        adapted to process the second digital input signal in accordance        with a predetermined second signal processing algorithm to        provide a second acoustical output signal; Thereby, the hearing        prostheses are operated in a time-synchronised manner so as to        provide a DSP based hearing aid system which supports matched        signal delays through the hearing prostheses.

According to this second aspect of the invention, spread spectrumtechnology is employed to synchronise the signal processing of thehearing prostheses by the transmitted synchronisation signal and basedon the repetitive coding sequence. By not transmitting bi-directionaldata signals during operation, power consumption within the wirelesstransceivers may be significantly reduced in both hearing aids.

The first DSP may, furthermore, be adapted to generate a digital controldata signal for controlling an operation mode of the second hearingprosthesis and the first wireless transmitter may be adapted to modulatethe digital control data with the repetitive coding sequence and use thedigital control data as the synchronisation signal. The control data arethus modulated with the repetitive coding sequence and transmitted tothe second hearing prosthesis where they are retrieved in a mannercorresponding to the retrieval of the first and second data signalsdescribed in connection with the first aspect of the invention.

The repetitive coding sequence provided by the first and second sequencegenerators of the binaural hearing system or by the sequence generatorsof the synchronised hearing aid system may comprise, or be constitutedby, a pseudo-random noise (PN) sequence. Alternatively, each sequencegenerator may be adapted to select a carrier frequency provided by afrequency synthesiser based on values of a pseudorandom noise (PN)sequence to generate a frequency-hopped repetitive coding sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a binaural hearing aid systemaccording to the invention,

FIG. 2 shows a simplified block diagram of an integrated DS-SStransceiver system for a hearing aid system according to the invention,

FIG. 3 is a more detailed block diagram of a receiver and a clockextraction and generating part of the DS-SS transceiver system shown inFIG. 2,

FIG. 4 is a block diagram that shows in more detail a circuit forgenerating a synchronised coding sequence,

FIG. 5 is a block diagram showing in more detail the clock VCO circuitof FIG. 4.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In the following, a specific embodiment of a DSP based hearing aidsystem according to the invention is described and discussed in greaterdetail. The present description discusses in detail only a wirelessDS-SS bi-directional communication system and its utilisation tosynchronise corresponding clock signals between two individual hearingaids of the system.

To support low power and low voltage operation of the present wirelessDS-SS communication system and associated DSPs, logic gates and otherdigital circuits are preferably implemented in a low threshold voltageCMOS process. Preferred processes are 0.5-0.18 μm CMOS processes withthreshold voltages located in the range from about 0.5 to 0.8 Volt.

In the overall system diagram of the binaural hearing aid system shownon FIG. 1, a first or master hearing aid 0 and a second, or slavehearing aid 0_, are communicating bi-directional data signals in atime-multiplexed mode. Each hearing aid comprises an associatedprogrammable DSP 2, 2a which processes respective input signals providedby oversampled analogue-to-digital converters 1b, 1c. Receivers 3, 3aconvert their respective processed data signals to respective acousticsignals perceivable for the hearing aid user. A circuit block 4comprises a master oscillator that generates a sampling clock signal forthe analogue-to-digital converters 1b and a clock signal for the DSP 2.The second hearing aid 0_ receives a digital data signal modulated byDS-SS spectrum method, further explained in connection with FIG. 2, andretrieves by a phase-locked or delay-locked loop 8 a synchronous clocksignal from a received first data signal transmitted by the masterhearing aid 0. The first data signal has been modulated by a synchronouspredetermined repetitive pseudorandom noise sequence. The retrievedsynchronous clock signal is utilised to derive a sampling clock signalfor the analogue-to-digital converter 1c and a DSP clock signal for theDSP 2a. Accordingly, clock signals for the oversampledanalogue-to-digital converters 1b, 1c and the DSP 2, 2a are locked toeach other to allow these devices to be operated synchronously.

In the simplified block diagram of FIG. 2, the transceiver of the first,or master, hearing aid is illustrated only in its transmission mode andthe transceiver of the second, or slave, hearing aid is illustrated onlyin its reception mode. However, it should be understood that in thepreferred embodiment of the invention, both transceivers of the presentbinaural hearing aid system comprises a transmitting part and areceiving part so that each transceiver alternates between transmittingthe digital data signal to the other side and receiving the digital datasignal from the other side in a full duplex time-multiplexed scheme. Thenumber of symbols or data bits of the digital input signals that it ispractical to transmit/receive in one “burst” will vary depending onspecific requirements to the binaural hearing aid system in question. Tokeep audio delay time low through the individual hearing aids of thesystem, it is preferred that 1-32 audio samples, or 16-512 symbols foran unencoded digital input signal of 16 bit samples, such as about 16audio samples are transmitted/received from/in each transceiver duringone “burst”. If the sampling rate (or decimated rate if an oversamplinganalogue-to-digital converter is utilised) of a microphone input signalis designed to about 16 kHz, a delay of 32 samples will correspond to adelay time of 2 ms, a value which is added to an inevitable inherentsignal delay time through each of the hearing aids of the system.

In FIG. 2, the first data signal is supplied at a terminal, Data In,from a DSP(not shwon) of the master hearing aid to a code modulator 5that modulates data bits or symbols of the first data signal byrespective consecutive 16 bit code sequences taken out of apredetermined repetitive pseudo-random noise sequence or PN sequence.Thereby, a first modulated data signal of the first hearing aid isformed on signal line 10 with a bit rate 16 times higher the originalrate, i.e. the bit rate of the first data signal, and a correspondinglybroader spectral bandwidth. The raised data rate of the first modulateddata signal on signal line 10 is by convention referred to as the“chipped-rate”. The first modulated data signal is further modulated upin frequency by an Radio Frequency (RF) modulator 15 before a compositeRF signal is transmitted to the second hearing prosthesis over antenna20. The frequency of the carrier of the RF modulator 15 is preferablyselected in the range 200 MHz-1 GHz. The length of PN sequence ispreferably about 2¹⁶-1 and each pair of hearing aids in the binauralhearing aid system is provided with its own unique PN sequence which issubstantially orthogonal to all other codes that may be used in otherhearing aid systems of the same type. Thereby, interference betweenclosely spaced hearing aid systems can be avoided because only hearingaids that belong to the same pair are able to acquire mutual lock andcommunicate the digital signals.

In the second hearing prosthesis, a second antenna 30 receives thecomposite RF signal transmitted by the first hearing aid. A RFdemodulator 35 downconverts the received composite RF signal to abaseband frequency range and extracts the first modulated data signal.Thereafter, a clock and data retrieval and generating circuit 40multiplies the first modulated data signal with an synchronous versionof that PN code that was used to encode the first data signal in thefirst hearing aid.

Since the product of two versions of a predetermined repetitivepseudorandom noise sequence or PN sequence is one only if the twoversions are exactly in phase, the clock and data retrieval andgenerating circuit 40, within the second hearing aid, is able to acquireand maintain lock to the transmitter by continuously evaluating anautocorrelation function between the two versions of the PN code andadjust a relative phase between the PN sequences to obtain a maximumcorrelation value. This issue will be addressed further in connectionwith the description of FIGS. 3 and 4. Finally, at an output terminal,Data Out, of the clock and data retrieval and generating circuit 40, anretrieved and synchronous version of the first data signal and aretrieved synchronous clock signal (not shown) has been obtained. Theretrieved synchronous clock signal is subsequently used to furtherderive appropriate synchronous clock signals for various parts of thesignal sampling and processing circuits of the second hearing aid. Ofparticular importance in this connection is the generation of asynchronous sampling clock signal (xx FIG. 1) that controls the samplingof the second aid's microphone input signal so as to be synchronous withrespect to the corresponding sampling of the microphone input signal ofthe first hearing aid.

In an alternative embodiment of the above-described integrated DS-SStransceiver system, the (traditional) RF modulator 15 and demodulator 35circuits have been designed to operate at communication frequency whichis very low compared to typical RF communication frequencies, e.g. lowerthan the above-mentioned 200 MHz-1 GHz RF communication frequency range.Such a low RF carrier frequency may be as low as only about 4-8 timeshigher than the chipped-rate of the modulated data signals, to furthersave power and reduce complexity of the transceivers. RF antennas 20 and30 has also been replaced by respective inductive coils adapted tocommunicate the first and second data signals between the first andsecond hearing aids by utilising near-field magnetic coupling betweenthe inductive coils. The requirement to transmission distance of abinaural hearing aid system is in the order of 15-25 cm. Theabove-described wireless magnetic coupling technique is practicalbecause of the short transmission distance. Furthermore, magneticallycoupled system, has as another attractive, a limited far-field emissionof electro-magnetic signals compared to the emission of traditionalfar-field coupled system which are obtained at higher communicationfrequencies and communicated over antennas designed to operate at suchhigher communication frequencies.

Consequently, instead of using traditional antennas, it may prove morepower efficient to transfer the digital data signals for hearings aidapplications, and other very short-range applications, by way ofmagnetic induction. Crucial issues are that the distance between thehearing aids is not much larger than physical dimensions of the coils,and that the physical dimensions of the coils are very small (at leastabout 10 times smaller) compared to the wavelength of the RF carrier.Under such conditions, the transmitter power required to transmit adesired bandwidth and at a sufficiently low bit error rate (BER) may betransferred by near-field magnetic coupling, or mutual induction, whileat the same time minimising far-field coupling. Minimising the far-fieldcoupling helps improving the interference immunity and compliance to EMCregulations in general.

The first and second data signals may be coded versions of digital audiosignals processed within the respective hearing aids, such as codedversions of the first and second digital input signals obtained from therespective microphone signals. The first and second data signals mayalso be constituted by digital signals that has been processed by theDSPs or the first and second data signals may represent unencodeddigital input signals. The coding may be provided to support errordetection and/or correction of the received digital signals according toa number of methods well known in the art, e.g. Reed Solomon coding.Encoding may further be applied for the purpose of removing any DCcontent of the digital signals prior to their transmission in order tosimply the design of the receiving part of the transceivers. Finally,the coding of the digital data signals may comprise the step ofinserting control data or information into the first and/or second datasignal(s) and extract these control data at the receiving side tocommunicate control information between the hearing aids.

The transmission frequency for the present near-field magneticallycoupled communication system is preferably selected in the range 50-100MHz and each inductive coil may have a inductance of between 200 nH and2 μH. The data or symbol rate of each of the first and second datasignals is preferably about 600 Kbit/s in order to support an audio rateof about 256 Kbit/s of each of the first and second data signals incombination with an effective transmission duty cycle of about 50% plusoverhead data for a forward error correction scheme. Accordingly, ifthese 600 Kbit/s first and second data signals are modulated with 16codes of the PN code sequence per data bit, the resulting chip rate ofeach of the modulated data signals will be about 9600 Kbit/s. If an evenhigher transmission frequency is desired, further RF modulation orup-conversion may be applied to the “chipped” modulated data signal inorder to further raise its transmission frequency to a desired, ortarget, range, as explained above. For the near-field magnetic coupledcommunication system, the further RF carrier frequency is preferablyselected to be only about 4-8 times higher than the chipped rate of themodulated data signals. An important advantage of operating theintegrated DS-SS transceiver system by near-field magnetic coupling isthat it may be possible to reduce the required transmission power to alevel that is below RF spurious emission requirements according tonational and/or international EMC norms. These spurious emissionrequirements are in practice measured in the far-field of the deviceunder consideration.

However, a near-field magnetic coupled communication system is capableof coupling more of the transmitter's emitted electromagnetic power tothe receiving antenna than a corresponding traditional RF basedcommunication system is capable of for any fixed level of far-fieldelectromagnetic power. Consequently, for the purpose of suppressing RFspurious emission power from the transceivers, as measured in thefar-field, the near-filed magnetic coupled system has superiorcharacteristics.

According to the European EMC norm EN55022 all radio transmittingdevices must have an emitted spurious power density of less than −54 dBmin most of the frequency range below 230 MHz and below −54 dBm from 230MHz-1 GHz. Consequently, if the emitted power density of the integratedDS-SS transceiver system is kept below −54 dBm everywhere in the 0 Hz-1GHz transmission frequency band, the transceiver system will be able tomeet these requirements.

In FIG. 3, the composite RF signal is amplified and bandpass filtered byRF input circuit 100. A RF carrier recovery circuit 105 extracts a RFcarrier from the composite RF signal, and the RF carrier is subsequentlymixed or multiplied with the composite RF signal by downconverter 110.The modulated data signal, constituted by the digital signal modulatedat the chip-rate, has now been recovered at an output of thedownconverter 110. Thereafter, the modulated data signal is applied to aPN signal synch and symbol timing circuit 115 that generates theretrieved synchronous clock signal that defines the symbol rate of thedigital signal and a retrieved synchronous “chipped” clock signal. Theretrieved synchronous clock signal is accordingly used to control anintegration time period of integrator 125 and an integrator outputsignal is applied to a decision device that converts the result of theintegration to a corresponding bit value, e.g. +1 or −1. Errorcorrection circuit 130 detect/correct any errors in the output signal ofthe decisions device and thereby provides the retrieved synchronousdigital signal at its output. The retrieved synchronous “chipped” clocksignal is used by a PN signal synch circuit to control a timing of alocal PN sequence generator 120 which generates the specific PN sequenceutilised by the pair of hearing aids in question.

FIG. 4 shows a delay-locked loop that has been designed to implement thePN signal synch and Symbol timing circuit (115, FIG. 3). The local PNgenerator 120 and two time-shifted versions of the synchronized PNsignal are used to generate early and late controls signal so as toadjust the phase of the synchronised sequnce signal to obtain maximumcorrelation between the local PN generator's signal and the retrievedmodulated data signal. The time shifts are plus and minus T_(v)/2respectively.

FIG. 5 shows in more detail a block diagram of the preferred clock VCO(200, FIG. 4) to illustrate a preferred acquisition method that uses aso-called sliding correlator. If the integrator (125, FIG. 3) outputfalls below a certain threshold for M consecutive symbols, the slidingcorrelator drops one clock cycle to the local PN sequence generator(120, FIG. 3). This will offset the sequence generated by the local PNgenerator one cycle. The PN signal is cyclic with a period of L, thatmay be selected between 2⁸-1 and 2¹⁶−1, and cycle to cycle alignment tothe transmitter's PN sequence will occur after up to L cycle steels.

1. A binaural hearing system comprising a first and a second hearingprosthesis adapted for wireless bi-directional communication of digitaldata signals; the first hearing prosthesis comprises: a first microphoneadapted to generate a first input signal in response to receivingacoustic signals, a first analogue-to-digital converter adapted tosample the first input signal by a first sampling clock signal togenerate a first digital input signal, a first clock generator adaptedto generate a coding clock signal, a data rate clock signal and thefirst sampling clock signal synchronously with respect to each other, afirst sequence generator adapted to generate a repetitive codingsequence synchronously to the coding clock signal, first data generatingmeans adapted to provide a first data signal synchronously to the datarate clock signal, a first wireless transceiver adapted to receive andmodulate the first data signal with the repetitive coding sequence totransmit a first modulated data signal to a second wireless transceiverof the second hearing prosthesis and to retrieve a second data signalfrom a second modulated data signal received from the second wirelesstransceiver, first output means adapted to convert a first processeddata signal to a first acoustical or electrical output signal; and thesecond hearing prosthesis comprises: a second microphone adapted togenerate a second input signal in response to receiving acousticsignals, a second analogue-to-digital converter adapted to sample thesecond input signal by a second sampling clock signal to generate asecond digital input signal, a second sequence generator adapted togenerate a version of the repetitive coding sequence of the firstsequence generator synchronously to a second coding clock signal, seconddata generating means adapted to provide a second data signalsynchronously to a retrieved clock signal, a second wireless transceiveradapted to receive the first modulated data signal from the firstwireless transceiver and to modulate the second data signal with theversion of the repetitive coding sequence to transmit a second modulateddata signal to the first wireless transceiver, second clock and dataretrieval means adapted to lock onto the first modulated data signal toretrieve the first data signal and to generate the second sampling clocksignal and the retrieved clock signal, synchronously to the first codingclock signal, by correlating said first modulated data signal with theversion of the repetitive coding sequence, second output means adaptedto convert a second processed data signal to a first acoustical orelectrical output signal; whereby the respective sampling clock signalsof the hearing prostheses are synchronised in time so as to provide ahearing system with synchronous sampling of the respective microphoneinput signals.
 2. A binaural hearing system according to claim 1,wherein the data generating means of the first hearing prosthesiscomprises a Digital Signal Processor adapted to process the firstdigital input signal and the second data signal in accordance with apredetermined signal processing algorithm to provide the first processeddata signal; or the data generating means of the second hearingprosthesis comprises a Digital Signal Processor adapted to process thefirst data signal the second digital input signal in accordance with apredetermined signal processing algorithm to provide the secondprocessed data signal.
 3. A binaural hearing system according to claim1, wherein the data generating means of the first hearing prosthesiscomprise: a first Digital Signal Processor adapted to process the firstdigital input signal and the second data signal in accordance with apredetermined first signal processing algorithm to provide the firstprocessed data signal to the first output means, and the data generatingmeans of the second hearing prosthesis comprise: a second Digital SignalProcessor adapted to process the second digital input signal and thefirst data signal in accordance with a predetermined second signalprocessing algorithm to provide the second processed data signal to thesecond output means.
 4. A binaural hearing system according to claim 3,wherein the first Digital Signal Processor and the first output meansoperate synchronously to the coding clock signal, and the second DigitalSignal Processor and the second output means operate synchronously tothe retrieved clock signal; whereby the acoustical or electrical outputsignals of the respective hearing prostheses may be synchronised in timeso as to provide a hearing system capable of delivering phase-alignedacoustic or electrical output signals to a user.
 5. A binaural hearingsystem according to any of the preceding claims, wherein the secondhearing prosthesis further comprise: a second clock oscillator adaptedto generate a second coding clock signal and the second sampling clocksignal, clock mode selection means operatively connected to the secondclock and data retrieval means and the second clock oscillator andadapted to selectively use the second clock and data retrieval means orthe second clock oscillator as a source for clock signals in the secondhearing prosthesis; thereby supporting a mono-aural operation mode ineach prosthesis during time periods with interruptions in the firstmodulated data signal.
 6. A binaural hearing system according to claim5, wherein the first hearing prosthesis further comprises first clockand data retrieval means allowing the prosthesis to lock onto the secondmodulated data signal to synchronise clock signals of the firstprosthesis to the second clock oscillator; thereby providing a binauralhearing system that allows the first or the second hearing prosthesis tooperate as a master device and the other as a slave device duringbinaural operation.
 7. A binaural hearing system according to claim 6,wherein each of the hearing prostheses comprise: a programming interfacefor exchanging programming data between a host programming system andthe hearing prosthesis, and a configuration register programmablethrough the programming interface and operatively connected to the clockmode selection means to control their operation; thereby supportingfitting session configurable system.
 8. A binaural hearing systemaccording to claim 1, wherein the first wireless transceiver furthercomprises: a first RF modulator adapted to further modulate the firstmodulated data signal to generate and transmit a first RF modulated datasignal to the second hearing prosthesis and a first RF demodulatoradapted to recover the second modulated data signal from a second RFmodulated data signal, and wherein the second wireless transceiverfurther comprises a second RF modulator adapted to further modulate thesecond modulated data signal to generate and transmit the second RFmodulated data signal to the first hearing prosthesis and a second RFdemodulator adapted to recover the first modulated data signal from thefirst RF modulated data signal from the first wireless transceiver.
 9. Abinaural hearing system according to claim 1, wherein each of the firstand second wireless transceivers comprises an inductive coil, theinductive coils being adapted transmit and receive the modulated datasignals or the RF modulated data signals by utilising near-fieldmagnetic coupling between said inductive coils.
 10. A wirelesssynchronised hearing aid system comprising a first and a second hearingprosthesis, wherein the first hearing prosthesis comprises: a firstmicrophone adapted to generate a first input signal in response toreceiving acoustic signals, a first analogue-to-digital converteradapted to sample the first input signal by a first sampling clocksignal to generate a first digital input signal, a first clock generatoradapted to generate a coding clock signal and a first sampling clocksignal synchronously with respect to each other, a first sequencegenerator adapted to generate a repetitive coding sequence synchronouslyto the coding clock signal, a first wireless transmitter adapted totransmit a synchronisation signal based on the repetitive codingsequence to a second wireless receiver of the second hearing prosthesis,a first Digital Signal Processor and first output means, operatedsynchronously to the the coding clock signal, and adapted to process thesecond digital input signal in accordance with a predetermined secondsignal processing algorithm to provide a first acoustical output signal;and the second hearing prosthesis comprises: a second microphone adaptedto generate a second input signal in response to receiving acousticsignals, a second analogue-to-digital converter adapted to sample thesecond input signal by a second sampling clock signal to generate asecond digital input signal, a second sequence generator adapted togenerate a version of the repetitive coding sequence of the firstsequence generator synchronously to a retrieved clock signal, the secondwireless receiver being adapted to receive the synchronisation signaland retrieve the repetitive coding sequence, second clock retrievalmeans adapted to lock onto the synchronisation signal to retrieve togenerate the retrieved clock signal and the second sampling clocksignal, synchronously to the first coding clock signal, by correlatingsaid synchronisation signal with the version of the repetitive codingsequence, a second Digital Signal Processor and second output means,operated synchronously to the retrieved clock signal, and adapted toprocess the second digital input signal in accordance with apredetermined second signal processing algorithm to provide a secondacoustical output signal; whereby the hearing prostheses are operated ina time-synchronised manner so as to provide a DSP based hearing aidsystem with matched signal delay through the hearing prostheses.
 11. Asynchronised hearing system according to claim 10, wherein the first isadapted to generate digital control data for controlling an operationmode of the second hearing prosthesis, and the first wirelesstransmitter is adapted to modulate the digital control data with therepetitive coding sequence and use the digital control data as thesynchronisation signal.
 12. A synchronised hearing system according toany of claims 10-11, wherein the repetitive coding sequence of the firstand second sequence generators comprises a pseudo-random noise (PN)sequence.
 13. A synchronised hearing system according to any of claims10-11, wherein the first sequence generator is adapted to select acarrier frequency of frequency synthesiser based on values of apseudorandom noise (PN) sequence to generate a frequency-hoppedrepetitive coding sequence.